Physiological effects of unequal alternating monocular exposure.

1. In order to investigate the effects of an imbalance in stimulation to the eyes without the confounding influence of continuous deprivation of one eye, we reared cats with unequal alternating monocular exposure (AME) and, for comparison, cats with equal AME. We recorded extracellularly from single cells in area 17 of visual cortex. 2. For unequal AME cats, a majority of the cells that were visually responsive were dominated by the eye that had received more patterned visual experience. The percentage of cells dominated by the more experienced eye was greater with a large imbalance in stimulation to the two eyes (AME 8/1, 77%) than with a small imbalance (AME 8/4, 62%). 3. For both equal AME cats and unequal AME cats, we obtained evidence for differences in cells activated by the contralateral and by the ipsilateral afferents. a) In equal AME cats receiving only 1 h of exposure per day, we obtained a greater dominance by the contralateral eye (60%) than in equal AME cats receiving 8 h of exposure per day (42%). b) Although a large imbalance in stimulation (AME 8/1) resulted in a shift in ocular dominance in both cortical hemispheres, a moderate imbalance (AME 8/4) resulted in a smaller shift, which was apparent only in the hemisphere ipsilateral to the less-experienced eye. 4. The percentage of cortical cells responsive to each eye was uniform throughout the depth of cortex. Thus, for the unequal AME cats, cells activated by the less-experienced eye were no more frequent in layer IV of visual cortex than in the infragranular and supragranular layers. 5. Although almost all cells recorded from AME cats had relatively normal receptive-field properties, three receptive-field properties of cells in unequal AME cats showed an effect of the rearing. In each case cells dominated by the less-experienced eye and recorded in the cortical hemisphere ipsilateral to it showed the largest changes. These cells a) were more poorly tuned, b) had lower cutoff velocities, and c) had smaller receptive fields. 6. It is suggested that cortical cells that putatively receive Y-cell afferents from the dorsal lateral geniculate nucleus (LGNd) are more affected by an imbalance in stimulation than are cortical cells that putatively receive X-cell afferents. Thus, the decrease in mean receptive-field area and cutoff velocity for the cells dominated by the less-experienced eye is suggested to be due to a greater shift in ocular dominance by the cortical cells receiving Y-cell afferents from the LGNd. 7. The interaction between binocular competition and deprivation of pattern vision may contribute to differences between monocularly deprived cats and unequal AME cats.